1. Field of the Invention
The invention relates to the addition of color mixing optics and optical feedback to produce uniform color throughout the output light beam of a multi-color linear LED illumination device.
2. Description of Related Art
Multi-color linear LED illumination devices (also referred to herein as lights, luminaires or lamps) have been commercially available for many years. Typical applications for linear LED illumination devices include wall washing in which a chain of lights attempt to uniformly illuminate a large portion of a wall, and cove lighting in which a chain of lights typically illuminates a large portion of a ceiling. Multi-color linear LED lights often comprise red, green, and blue LEDs, however, some products use some combination of red, green, blue, white, and amber LEDs.
A multi-color linear LED illumination device typically includes one or more high power LEDs, which are mounted on a substrate and covered by a hemispherical silicone dome in a conventional package. The light output from the LED package is typically lambertian, which means that the LED package emits light in all directions. In most cases, Total Internal Reflection (TIR) secondary optical elements are used to extract the light emitted from a conventional LED package and focus that light into a desired beam. In order to extract the maximum amount of light, the TIR optics must have a specific shape relative to the dome of the LED package. Other dimensions of the TIR optics determine the shape of the emitted light beam.
Some multi-color linear LED light products comprise individually packaged LEDs and individual TIR optics for each LED. In order for the light emitted from the different colored LED emitters to mix properly, the light beams from each individual color LED must overlap. However, because the LEDs are spaced centimeters apart, the beams will overlap and the colors will mix only in the far field, at some distance away from the linear light. At a very close range to the linear light, the beams will be separate and the different colors are clearly visible. Although such a product may exhibit good color mixing in the far field, it does not exhibit good color mixing in the near field.
Other multi-color linear LED light products use red, green, and blue LEDs packaged together with a single TIR optic attached to each RGB LED package. These RGB LED packages typically comprise an array of three or four LEDs, which are placed as close together as possible on a substrate and the entire array is covered by one hemispherical dome. In products that use one TIR optical element for each multi-color LED package, there is not necessarily a need for the beams from the different TIR optical elements to overlap for the colors to mix. Therefore, such products tend to have better near field color mixing than products that use individually packaged LEDs.
However, depending on the size of the primary and secondary optics, the far field color mixing may actually be worse in products that package multiple colors of LEDs together. Since the different colored LEDs are in physically different locations within the hemispherical silicone dome, the light radiated from the dome, and therefore, from the TIR optical element will not be perfectly mixed. Although larger domes and larger TIR optical elements may be used to provide better color mixing, there are practical limits to the size of these components, and consequently, to the near and far field color mixing provided by such an approach.
An alternative optical system, although not commonly used, for color mixing and beam shaping in multi-color LED linear lights uses reflectors. In some cases, the light from a plurality of multi-colored LED emitter packages are mixed by a diffusion element and shaped by a concave reflector that redirects the light beams down a wall. The diffusion element could be combined with an exit lens or could be a shell diffuser placed over the multi-color emitter packages, for instance. Alternatively, the system could use a shell diffuser and a diffused exit lens. Although such systems can achieve very good color mixing in both the near and the far field, there is a tradeoff between color mixing and optical efficiency. As the amount of diffusion increases, the color mixing improves, but the optical efficiency decreases as the diffuser absorbs and scatters more light.
As LEDs age, the light output at a given drive current changes. Over thousands of hours, the light output from any individual LED may decrease by approximately 10-25% or more. The amount of degradation varies with drive current, temperature, color, and random defect density. As such, the different colored LEDs in a multi-color LED light will age differently, which changes the color of the light produced by the illumination device over time. A high quality multi-color LED light that can maintain precise color points over time should have the means to measure the light output from each color component, and adjust the drive current to compensate for changes. Further, a multi-color linear light should have the means to measure the light produced by each set of colored LEDs independent from other sets to prevent part of the linear light from producing a different color than other parts.
Multi-color LED linear lights with TIR optics on each individual LED cannot achieve good color mixing in the near field. Multi-color LED linear lights that combine a multi-color LED package with a TIR optical element require a large TIR optical element to achieve good color mixing in the near and far fields. Multi-color LED linear lights that use conventional diffusers and reflectors to achieve good color mixing in both the near and the far field suffer optical losses. As such, there is a need for an improved optical system for multi-color LED linear lights that provides good color mixing in the near and far fields, is not excessively large and expensive, and has good optical efficiency. Further, there is a need for an optical feedback system to maintain precise color in such linear lights. The invention described herein provides a solution.